In recent years, materials have been developed that demonstrate a change in their optical properties on exposure to specific types of radiation. For example, some glass materials demonstrate a change in their refractive index after exposure to actinic radiation. Doping of glass fibers with germanium is one way to make them responsive to actinic radiation so that their localized refractive index can be changed.
The ability to change the optical properties of these materials, and in particular their refractive indices, has become important in numerous applications. One such application is creating gratings in optical fibers, which are regions in an optical fiber having periodic or quasi-periodic variations in refractive index. These fiber gratings can sometimes be thought of as a series of adjacent parallel planes of alternating higher and lower refractive index. Gratings have a number of important applications, including use as very narrowband retroreflectors suitable for providing feedback at a specific wavelength in fiber lasers (both in short pulse and single frequency lasers), as gain flattening devices in optical amplifiers, and as filters for multichannel wavelength-division multiplexed (WDM) communications systems.
Gratings are generally classified into two groups, long period and short period (or Bragg) gratings. Long period gratings scatter light into forward propagating cladding modes. Bragg gratings reflect light into counter propagating core (or cladding) modes.
If the spacing of the grating planes is varied across the length of the grating it is possible to produce a chirped grating, in which different wavelengths can be considered to be reflected from different points along the grating. Such gratings can be used to provide light dispersion, either to compensate for fiber dispersion in fiber links, or to manipulate optical pulses, as in a chirped pulse amplification (CPA) system.
During manufacture of optical glass fibers, the glass fibers are traditionally coated with a polymeric material to protect and maintain the intrinsic strength of the fiber during handling. The term “coating” generally refers to a material that is first applied to a solid substrate in a liquid state, then solidified by UV radiation (photopolymerizable), heat (thermoset), or by removing solvent molecules from the coating solution. In order to make a quality Bragg grating in these fibers it is usually necessary to remove the protective coating. The coating is normally removed by an acid bath. This is followed by formation of the grating and application of a new coating. This multi-step method of removing the coating, modifying the fiber, and then recoating the fiber can be time consuming, expensive and may result in a reduction in the strength of the fiber.
These steps are necessary for most applications because the gratings can not normally be formed through the coatings covering the fiber. Gratings cannot normally be formed through coatings for a number of reasons. First, the coatings often have a variable thickness, and this variable thickness can create a distorting lens that alters the path of the actinic radiation, resulting in a less precisely formed grating. Any lack of homogeneity, surface irregularities, or other optical imperfections can also degrade the quality of Bragg gratings written through such coatings. Second, although some coatings are highly transparent, they still often partially absorb the actinic radiation and overheat or are degraded by the high doses of radiation energy typically needed to form Bragg gratings in photosensitive glasses. In some circumstances, irradiation can actually result in the coating being degraded (such as by being charred) or ablated from the fiber.